Conventional gearboxes comprise a main shaft, a counter shaft, a plurality of gearwheel pairs each comprising a primary gearwheel fixedly arranged on the countershaft and a secondary gearwheel rotatably arranged on the main shaft. The primary gearwheel and the secondary gearwheel of the gearwheel pairs are constantly in meshing engagement with each other. The main shaft may support synchronizing units arranged at the vicinity of the secondary gearwheels of the respective gearwheel pairs. The synchronizing units are used to synchronize and lock the secondary gearwheels on the main shaft when a gear is to be engaged in the gearbox. The synchronizing units comprises friction surfaces which are moved into contact with each other when the speed of a secondary gearwheel is to be synchronized with the speed of the main shaft. The friction surfaces of the synchronization units are exposed to heavy loads and risk wear related problems.
An alternative to synchronizing units, it is to use a brake which reduces the speed of the counter shaft during an upshift process. Such a brake reduces the speed of the counter shaft until a secondary gearwheel receives a synchronous speed with the main shaft whereupon a locking unit provides a rotational locking of the secondary gear on the main shaft. During a downshift process, a drive motor provides an acceleration of the counter shaft, via an input shaft to the gearbox, until the secondary gearwheel receives a synchronous speed with the main shaft whereupon a lock unit provides a rotational locking of the secondary gear on the main shaft.
Gearboxes of heavy vehicles are often provided with a PTO (Power Take-Off).
The PTO may comprise a shaft which, at one end, is connectable to the counter shaft in the gearbox. The PTO shaft is at an opposite end connected to a power take-off connection usually arranged on the same side of the gearbox as the output shaft of the gearbox. It is many times desired to use the PTO when the vehicle is moving. However, it is not possible to shift gear in the gearbox when the PTO is connected to the counter shaft due to high torque or inertia of the engaged PTO. In a simple kind of PTO, the PTO shaft is to be connected to the counter shaft when the vehicle is stationary. In this kind of PTO, it is only possible to use the PTO with the start gear in the gearbox. In a more advanced PTO, it is possible to accelerate the vehicle to the speed at which the PTO is to be used. Thereafter, the speed of the PTO shaft and the counter shaft is synchronized by some kind of synchronizing unit whereupon the PTO shaft is connected to the counter shaft. In this case, it is possible to use the PTO with a higher gear in the gearbox.